Generally, polarizing plates used in image display apparatuses require having both of a high transmittance and a high degree of polarization to provide an image, which is bright, with excellent color reproduction characteristics. In general, such polarizing plates are produced by dyeing a conventional polyvinyl alcohol (PVA)-based film using dichroic iodine or dichroic dyestuffs and orienting the film by uniaxially stretching.
In the past, image display apparatuses employing a polarizing plate were generally used in small products such as electronic calculators and electronic watches. However, recently, image display apparatuses are used in almost all industrial fields such as televisions, monitors, dashboards, and display boards of all kinds of machines for office or industrial uses. According to increase of an area where image display apparatuses are used, the number of times when using image display apparatuses in a harsh environment such as high temperature and high humidity in a long time becomes increased. Accordingly, to show functions thereof in such harsh environments, it is required to develop a polarizing plate having an excellent thermal stability and durability.
As conventional methods used for improving a durability of a polarizing plate, there are a method of modifying a PVA-based film itself and a method of using dichroic dyestuffs that is not sublimate instead of an iodine-based polarizer that is sublimate.
In the case of the method of modifying a PVA-based film, a degree of polarization may be decreased since iodine or dichroic dyestuffs are not fully adsorbed onto a polymer matrix due to a modification of PVA or transmittance may be decreased due to a modification of a matrix. In the case of the method of using dichroic dyestuffs, a sufficient degree of polarization may not be obtained since it is difficult to control orientation while stretching PVA.
On the other hand, as another method of improving a durability of a polarizing plate, Japanese Patent Laid-Open Publication No. 2002-35512 discloses a method of producing a polarizer using a PVA film containing an appropriate amount of zinc. According to the method, when producing a polarizer by adding zinc, a durability of a polarizing plate is improved, thereby preventing red discoloration of cross nicols, a polarization defect of a long wavelength light, occurring at high temperature. However, when drying the polarizer, a boric acid is educed, thereby causing external defects.